Thiosquaramide-Based Supramolecular Polymers: Aromaticity Gain in a Switched Mode of Self-Assembly

Abstract

Despite a growing understanding of factors that drive monomer self-assembly to form supramolecular polymers, the effects of aromaticity gain have been largely ignored. Herein, we document the aromaticity gain in two different self-assembly modes of squaramide-based bolaamphiphiles. Importantly, O → S substitution in squaramide synthons resulted in supramolecular polymers with increased fiber flexibility and lower degrees of polymerization. Computations and spectroscopic experiments suggest that the oxo- and thiosquaramide bolaamphiphiles self-assemble into “head-to-tail” versus “stacked” arrangements, respectively. Computed energetic and magnetic criteria of aromaticity reveal that both modes of self-assembly increase the aromatic character of the squaramide synthons, giving rise to stronger intermolecular interactions in the resultant supramolecular polymer structures. These examples suggest that both hydrogen-bonding and stacking interactions can result in increased aromaticity upon self-assembly, highlighting its relevance in monomer design.

Thiolumazines as Heavy-Atom-Free Photosensitizers for Applications in Daylight Photodynamic Therapy: Insights from Ultrafast Excited-State Dynamics

“…To address these issues, the past decade has seen a substantial use of heavy-atom-free photosensitizers (HAFPSs), which are promising over other halogen and heavy-metal-based PSs. − This heavy-atom-free approach mainly comprises the replacement of carbonyl oxygen with a sulfur atom. The substitution of sulfur in place of oxygen provides an upper hand in understanding the conformational rigidity of peptide backbones through H-bonding, improved aromatic stability, , and tunable spectroscopic profiles . The seminal contributions from Crespo Hernández’s group on developing HAFPSs based on thiocarbonyl modification in nucleobases paved the path in designing novel PSs for photodynamic application. , They offer an (I) easy, synthetic, and cost-effective procedure, (II) biocompatibility for minimal side effects, (III) longer wavelength absorption for deeper tissue penetration, (IV) larger two-photon absorption cross section for less dark cytotoxicity, and (V) high triplet harvesting, i.e., near unity triplet quantum yield as a result of a small Δ E ST gap with high spin–orbit coupling (SOC).…”

Controlling Tautomerization in Pyridine‐Fused Phosphorus‐Nitrogen Heterocycles

“…Computed nucleus independent chemical shifts (NICS) [26.27] suggest that, in the solid state, the hydrogen bonded PN‐heterocycles adopt a dominant quinoidal form as a result of aromaticity gain. Similar effects of hydrogen bonding assembly on aromaticity gain are known and have been reported [28–32] . Isotropic NICS(0) values were computed at each of the six‐membered ring centers of the PN‐heterocycles, and the sum of NICS(0) values were compared to quantify the aromatic character of the tautomers.…”

“…Similar effects of hydrogen bonding assembly on aromaticity gain are known and have been reported. [28][29][30][31][32] Isotropic NICS(0) values were computed at each of the sixmembered ring centers of the PN-heterocycles, and the sum of NICS(0) values were compared to quantify the aromatic character of the tautomers. Here, we applied the NICS(0) index so that computed data for 2 a and 2 a' could be compared directly to isotropic NICS(0) data computed for a tetramer…”

A concise review on application of squaric acid derivatives and their metal complexes in medicine

“…They are ditopic hydrogen-bonding synthons that can self-associate through two acceptors (C=O groups) and two donors (N−H groups) of hydrogen bond directly opposite one another on a cyclobutenedione ring. [34][35] Squaramides are used as coupling units for chelator and targeting vectors in radiopharmaceuticals for diagnostic and therapeutic purposes in nuclear medicine. 36 Chiral squaramides are powerful bifunctional hydrogen-bonding catalysts, and promoted numerous catalytic asymmetric transformations.…”